TREELIM – Growth and stress physiology

C1 Freezing resistance

Freezing resistance is species specific, and the tolerance to extremely low temperatures during the critical late winter to early spring period reflects the elevational ranking of species and also matches the absolute minima of temperatures obtained from weather stations (past +30 years) when scaled to site elevation (this result derived form a preliminary model before our own temperature data are completely analysed). During budbreak all species dramatically lose their frost resistance and thereafter the resistance to freezing stress is very similar between species. Therefore the timing of budbreak is decisive for freezing resistance in spring. We developed routines that permit scanning large amounts of samples for freezing resistance by applying a close to nature freezing temperature progress, with minimum delay to field collection of samples. We also developed laboratory (histology) and statistical tools (LT 50) to process results (A. Lenz and G. Hoch presented oral conference presentations and posters).

Stress survival of broadleaved tree species could be one possible physiological explanation for the cold distribution limit of broadleaved tree species. In this case, low temperature extremes like freezing temperatures would govern the distribution patterns. Our aim was to find critical low temperatures directly affecting survival during bud break, when trees are very active and late frost events are still likely to occur. In addition, we want to explore adaptation of broadleaved tree species to freezing temperature. The specific questions are: Is survival – i.e. freezing resistance – the physiological factor determining the upper elevational limits of broadleaved tree species? Are broadleaved tree populations growing close to their upper elevational limits more resistant to freezing temperature than populations growing at lower elevation?

First, a new freezing laboratory has been established. This includes computer controlled freezers allowing for synchronised freezing to a series of parallel degrees of frost. A total of 4800 buds and leaves from 8 different species were collected in the western Swiss Alps, exposed to the freezing treatment and processed. After the freezing treatments, survival of plant material was assessed visually and by the electrolyte leakage method – an objective method to assess survival of buds. In winter 2012 (in progress), the same team investigated freezing resistance of the same seedlings as in the common gardens, representing different provenances, but grown in an extra garden in Basel to search for genetic adaptations to frost stress. These new data are complemented by frost resistance of adult trees growing in situ at low and high elevation in the western Swiss Alps. So far, a total of 5300 buds of 8 different broadleaved tree species belonging to maximum of 4 different provenances were collected, and exposed to controlled freezing. This work is still ongoing, and will provide data for two or more articles.

First results: All broadleaved tree species rapidly lose their frost resistance during bud break and show a very similar frost resistance from phenological stage 3 onwards, irrespective of the upper elevational limit. The maximum difference in frost resistance among species when the buds are closed is higher than 15 K, about 10 K when buds start swelling and opening (phenological stages 1 and 2). Then, as soon as the leaves are out, (stage 3 and 4), this difference is lower than 4 K. While the new leaves get more mature, frost resistance slightly increases, but the small differences in frost resistance among species remain constant. So phenology of species strongly influences frost resistance in spring. Long-term minimum temperature records show that Acer, Fagus and Fraxinus are the species that have the greatest risk to be damaged by late frost events during bud break where they grow.

The right timing of bud break and therefore survival of freezing stress during bud break, when trees are very active and late frost events are likely, is possibly one key driver of broadleaved tree species distribution. The proper mechanism of tree distribution – i.e. frost resistance during bud break – should be included in distribution, migration and climate change models, since it is a very strong driver.

These are largely unpublished, in part preliminary results, which are the intellectual property of the TREELIM team, and are not eligible for quotation.